1. Field of the Invention
The present invention is related to methods for manufacturing micro cavities or enclosures at the wafer level using MEMS (Micro-Electro-Mechanical Systems) processes, specifically to pressure sensors and hermetic or vacuum packaging of electronics devices.
2. Description of Related Art
With increasing developments in Micro-Electro-Mechanical Systems (MEMS), miniature sensors are filling the horizon. However, significant deterrents to military and commercial application of many of these devices exist. Of primary concern for almost all MEMS sensors to date is the issue of packaging where they must be packaging in vacuum or hermetic environment. The vacuum or hermetic packaging often takes more than 50% of the total cost of a MEMS device. This is especially true for vacuum packaging, which is used for packaging inertial sensors (for lower Brownian noise introduced by gas molecules), infrared detectors (for heat insulation), and radio-frequency (RF) MEMS devices such as RF switches and mechanical resonators.
Wafer-scale packaging, where all the devices on a wafer is packaged at the same time, is the key to lowering the cost of vacuum packaging for MEMS devices as all the sensors on the wafer are packaged at the same time. A prevalent approach to wafer scale packaging is to bond a capping wafer, which have cavities, to the device wafer, under vacuum, to seal individual MEMS chips on the device wafer in the cavities. The bonding may be done with glass-to-silicon anodic bonding, with electroplated solder, with screen-printed low melting-point glass frit, or with preformed Au—Sn. To prevent out-gassing that occurs with time and degrades the vacuum, metal gas getters are often used to absorb the out-gassed gas while spacers may be needed to control the gap between the wafers.
Several other wafer bonding techniques can be used for wafer level packaging. They include:
(1) direct silicon wafer bonding at high temperatures;
(2) Pyrex-silicon anodic bonding;
(3) silicon-silicon anodic bonding with sputtered intermediate Pyrex layers;
(4) silicon-silicon bonding using intermediate boron glass layers; or
(5) low-temperature wafer bonding using sodium silicate or aluminum phosphate intermediate thin layers.
These techniques, however, have the following drawbacks:
(1) Direct silicon wafer bonding requires high temperatures (800-1100.degree. C.) that is detrimental to most microelectronic devices;
(2) Glass-silicon anodic bonding are restricted to glass substrates which has different coefficient of thermal expansion (CTE) than silicon and can cause deformation after bonding;
(3) Anodic bonding requires a high DC voltage that can cause threshold voltage shift in the CMOS transistors;
(4) The intermediate boron glass layers are difficult to deposit and to etch; and
(5) Anodic bonding requires very clean and flat surfaces in the wafers, which make metal feedunders difficult to implement.
Micro cavities fabricated with MEMS technology have been used as pressure sensors, where one side of the cavity's enclosure is a thin membrane, which deflects if the pressures on two sides of it are different, and the opposite side is a rigid substrate. There is a gap g.sub.o between the membrane and the substrate. The principle of operation for typical pressure sensors is depicted in FIG. 1 (PRIOR ART). The strain 1040 in the membrane or the variation of gap thickness 1020 is a direct consequence of the membrane deflection 1010 caused by pressure 1000. These are two parameters that can be detected 1030 by a sensing device. In general these pressure sensors can be divided into four categories in terms of their sensing mechanisms: Piezoelectric, Piezoresistive, Capacitive and Optical. Most of the commercial pressure sensors are piezoresistive sensors. Silicon and germanium show greater piezoresistive effect than metals. Polysilicon and amorphous silicon also exhibit a strong piezoresistive effect. The sensing resistors are typically P-ion implantations in a n-type substrate. These resistors are diffused into the areas of high strain for maximum sensitivity. The basic structure of a piezoresistive pressure sensor consists of four sense elements in a Wheatstone bridge configuration to measure strain within a thin silicon membrane.
For the capacitive pressure sensors, capacitive sensors are used for gap thickness measurement. Deflection of one of the membranes/electrodes would modify the capacitance, which results in a variation of capacitance versus pressure. Absolute pressure sensors, which measures the difference between zero pressure (a perfect vacuum) and some known pressure, uses vacuum in the cavity to provide the absolute pressure reference. Many MEMS piezoresistive pressure sensors employ silicon-to-glass wafer bonding, which bonds a silicon wafer having bulk micromachined cavities and diaphragms, to a glass wafer under vacuum, to form the vacuum cavities.
Fabrication of capacitive pressure sensor is compatible with CMOS, thus it is more suitable for SoC architecture. Unfortunately, the fabrication process for such integrated pressure sensors has been very complex and expensive, thus it has not seen popularity. This technique is not compatible with CMOS as high voltage and temperature are involved. Polysilicon membrane enclosed vacuum cavities have been used in both piezoresistive and capacitive pressure sensors, their fabrication process also involves high temperatures.
Gas getter is normally used in vacuum packages to getter gases for preventing degradation of the vacuum with time. One example of gas gettering is barium metal used in vacuum tube or cathode ray tube that is evaporated from an electrical heating filament to the wall, when gas molecules in the vacuum chamber react with the metal and is trapped on the wall. A common way to getter gas in a vacuum package is electrically fired getter, where the getter is attached to an electrical heater and placed inside the package. It is heated up by passing electricity through the heater to activate the getter. Another method of activating the getter is to heat the entire vacuum package to a certain required temperature for a certain require period of time. Such getter is a metal mixture prepared in thick film format, so it cannot be deposited and thus must be placed inside the vacuum package individually by hand or tools. This is laborious and requires substantial amount of space and, thus, is undesirable for small packages. A getter that allows it to be deposited in thin film format is needed for batch processing. In addition, one that does not require high temperature activation is also needed.